The Conservation of the Endemic Vascular Flora of Ascension Island and Threats from Alien Species

Oryx Vol 39 No 4 October 2005

Short Communication The conservation of the endemic vascular flora of Ascension Island and threats from alien species

Alan Gray, Tara Pelembe and Stedson Stroud

Abstract We present the first assessment of the purpurascens, A. ascensionis, and X. ascensionensis appear endemic vascular flora of Ascension Island for over to be under no immediate threat of extinction, although 20 years. Combining new with existing data the follow- long-term survival is uncertain. E. origanoides, P. adscen- ing IUCN Red List categorizations are proposed: sionis, and S. caespitosus require active conservation Anogramma ascensionis, Dryopteris ascensionis, Oldenlandia measures to ensure their survival. adscensionis and Sporobolus durus (Extinct); Pteris adscensionis and Euphorbia origanoides (Critically Endangered); Keywords Sporobolus caespitosus (Vulnerable); Asplenium ascensionis, Ascension Island, exotic species, flora, Red Marattia purpurascens and Xiphopteris ascensionensis (Near List. Threatened). The primary cause of the four presumed extinctions appears to be introductions of exotic species This paper contains supplementary material that can and subsequent alteration of ecological conditions. M. only be found online at http://journals.cambridge.org

Ascension Island (97 km2) is a volcanic island in the is often shrouded in mist and a high proportion of the South Atlantic Ocean (7°57’S, 14°22’W). Its closest neigh- rainfall is in the form of occult deposition. The low-lying bour is St Helena 1,127 km to the south. Approximately lava plains exhibit xeric conditions common to many 1 million years old (Nielson & Sibbett, 1996), a distinctive desert habitats, and higher areas on the lava plains, such flora has evolved on Ascension, in common with many as Sisters Peak, remain dry for much of the time. other oceanic islands. The last Red List assessment of the Studer (1889 cited in Duffey, 1962) distinguished three vascular flora of Ascension (Cronk, 1980) concluded that vegetation zones, elaborated by Duffey (1962). Although of the 10 endemic species, 1 was Extinct, 5 Endangered the details have changed since the visits of both Studer and 4 Rare. The principal cause of the extinction and and Duffey, indicating Ascension’s vegetation ecology threats were 19th century introductions of exotic species to be in a state of flux, the zones remain useful for (Cronk, 1980). As the island is still in a state of ecological descriptive purposes. flux, a reassessment of the Red List status of the flora is Zone 1 Dry area below 330 m. Similar to Duffey’s long overdue. description, with patchy but often extensive vegetation, The climate of Ascension largely reflects its altitudinal especially after rain, with species including the endemic gradient but is modified by the south-east trade winds Euphorbia origanoides L., the native Aristida adscensionis (Duffey, 1964). On the coast at Georgetown temperatures L. and Cyperus appendiculatus Kunth, and the introduced are 27–31°C and mean annual rainfall c. 140 mm; tem- Enneapogon cenchroides (Licht.) C.E.Hubb., Argemone peratures at 660 m on Green Mountain are 6°C lower than at Georgetown and mean annual rainfall is mexicanca L., Heliotropium sp., Nicotiana glauca R.Grah., c. 680 mm (Ashmole & Ashmole, 2000). Green Mountain and Waltheria indica L.. The most notable change is the addition of Prosopis juliflora (Sw.) DC., which dominates large areas and has displaced much of the Acacia scrub Alan Gray (Corresponding author) Ecology and Resource Management, described by Duffey. School of GeoSciences, University of Edinburgh, Darwin Building, The King’s Buildings, Mayfield Road, Edinburgh, EH9 3JU, UK. Zone 2 Between 330 and 660 m. A more complete E-mail [email protected] coverage of vegetation with patchy mosaics of P. juliflora, Juniperus bermudiana L., Causurina equisetifolia L., Tecoma Tara Pelembe and Stedson Stroud Ascension Island Conservation, Conservation Centre, Georgetown, Ascension Island, ASCN IZZ, stans (L.) H.B. & K., Opuntia sp., Leucaena leucocephala South Atlantic Ocean. (Lam.) de Wit, Lantana camara L., N. glauca, Psidium

Received 15 June 2004. Revision requested 4 November 2004. guajava L., and Acacia spp.. This is the zone where the Accepted 13 April 2005. grass Melinis minutiflora attains its highest abundance.

Zone 3 Mist region from 660 to 850 m. This zone has Ascension are to expand their range, some control will been erroneously described as rainforest (Pearce, 2004). be required for all the severe threat species. The vegetation forms an almost complete cover made up Little is currently known about the biology and of a mosaic of grassland, scrub, woodland, bamboo (on ecology of the endemic species. Studies on the flora the summit) and ginger. Species in this mosaic include have been sporadic (Duffey 1964; Cronk, 1980; Gray et al., Alpinia zerumbet (Pers.) B.L.Burtt & R.M.Smith, Bambusa 2000) and have concentrated on distribution rather than sp., J. bermudiana I spp., Sporobolus africanus (Poir.) ecology or practical conservation. Although an ecological Robyns & Tournay, and Paspalum spp. investigation of E. origanoides was commissioned in 2003 Further details of the physical characteristics of Ascen- there are currently, to our knowledge, no plans to repeat sion Island are provided by Duffey (1964) and Ashmole this type of work for any of the other endemics. Details of & Ashmole (2000), and fuller discussions of flora, ecology the population dynamics of the endemics is limited to and conservation by Duffey (1964), Cronk (1980, 2000), anecdotal information. The estimated total area covered Packer & Packer (1997), Ashmole & Ashmole (2000), by populations of endemic species in 2003 appears to be Gray et al. (2000) and Gray (2003). greater than that given by Cronk (1980), but although Here we examine the threats to the flora of Ascension this may represent a real increase it could also reflect the (Gray, 2004; Wilkinson, 2004) and reassess the Red List greater recording effort of later surveys. It is difficult status of the endemic species (Walter & Gillet, 1998) to assess population dynamics without consistent and using the most recent criteria (IUCN, 2001). There is no standardized monitoring. All endemic populations are plant checklist for Ascension and taxonomic nomencla- localized and fragmented and, in the case of P. adscen- ture therefore follows Cronk (1980, 2000). The histori- sionis, there are few individuals remaining (Table 1). cally recorded sites of all the endemic vascular plants The best information available for any population is were surveyed in 1998, 2002 and 2003, and with a brief for E. origanoides. The increase in area occupied by the visit in November 2004 to assess E. origanoides. Survey species from 1958 (50 ha) to 1976 (75 ha) was in part work was broadened in 2002 and 2003, covering almost because of the appearance of a new population at South all of the island with the exception of inaccessible cliffs. Gannet Hill, now holding c. 90% of the island population. This provided estimates of population size and island The decrease from 1976 to 1998 (20 ha) seems to have distribution, and recorded any threats, including evi- been a combination of the loss of the English Bay popula- dence of habitat change and associated alien species. tion because of construction work and the effects of drier Introduced species that are capable of displacing an weather conditions. The rise from 1998 to 2003 (24 ha) endemic by dominating a particular site and altering eco- coincided with wetter climatic conditions (192 mm in logical conditions, such as light, nutrients and moisture 2003 compared to 97 mm in 1998). Population fluctua- availability, were categorized as severe threat species tions seem to be characteristic of this species, partly (Table 2). This was done with reference to a range of accounted for by rainfall events (Gray, 2003). A previ- sources but all are included in the SEPASAL database ously unrecorded population of c. 1,000 individuals near (Royal Botanic Gardens, Kew 1999), Cronk & Fuller Wig Hill was recorded in 2003. This is important because (2000) and Weber (2003). the population at South Gannet Hill suffered a 50% The flora of Ascension comprises c. 25 native vascular reduction (pers. obs., November 2004), perhaps because plants of which 10 are endemic (Cronk, 1980) (Table 1), and c. 280 introduced species (Appendices 1–2). Many of expansion of the rabbit population following feral cat introduced species are present at locations that have control; recent analysis of rabbit faeces found fragments endemics (Table 1), and most of these sites contain severe of E. origanoides (pers. obs., November 2004). threat species (Table 2). Of the 13 severe threat species, The most significant change in Red List status is the P. juliflora represents a threat not only to native plants categorization of Sporobolus durus, Dryopteris ascensionis but also to turtle nesting beaches (Fowler, 1998; Pickup, and Anogramma ascensionis as Extinct (Table 1). The only 1999). Control of this species has been discussed else- places where they could still exist would be in a remote where (Fowler, 1998) but the future of E. origanoides ravine or on a cliff face. Reported sightings of D. (especially for any restoration projects) may depend on ascensionis (1975) and O. adscensionis (in the 1980s) remain the extermination of P. juliflora in specific areas and unconfirmed (Cronk, 1980, 2000). The categories of Asple- containment of the feral sheep and donkey populations nium ascensionis, Marattia purpurascens and Xiphopteris that disperse seeds. Goats have also been implicated in ascensionensis are unchanged as their populations appear the extinction of at least one endemic species, Oldenlandia to be large enough for survival, at least in the short-term. adscensionis (DC.) Cronk, although goats are no longer Sporobolus caespitosus has been categorized to a higher present on the island (Cronk, 1980, 2000; Ashmole & threat category than these three species because of the Ashmole, 2000). If the extant endemic species on threat of encroachment of surrounding exotic vegetation.

Downloaded from https://www.cambridge.org/core. IP address: 170.106.35.229, on 25 Sep 2021 at 20:46:44, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605305001092 Flora of Ascension Island 451 ) native speciesspecies exotic 2 11 10 12 17 12 22 12 13 13 13 11 12 < < < < < < ee IUCN (2001) for an explanation of the criteria), justifica- No. localities at species (Table 2) are present, total area covered by each species, und. 5,000 (7) 16 5,000 (4) 9 500 2 (1) . 23,000(10) 18 . 1,000(5) 9 c c > 150–200 (5) 7 > > . Alpinia a severe threat; cappileus-veneris

P. juliflora . may be prone to long-term Adiantum Justification Est. pop. size threat species) (km population highly localized, 93% in one area, vulnerable to catastrophic events. population behaviour appears ephemeral; inferred from information in Hooker (1867), Duffey (1964) & Cronk (1980); vulnerable to catastrophic events long-term threats such as encroachment by inferred from information in Hooker (1867), Duffey (1964) & Cronk (1980); future bleak without population restoration programme. change and/or encroachment by introduced species, e.g. zerumbet threats such as habitat change and/or encroachment by introduced species. 1 B3c) Population may be unstable; apparent + status (criteria) 1 1998 Red 2004 Red List (no. with severe Total areaassociated No. associated No. Rare NT Localized but large population; no (DC.) EX EX Last seen 1889 (Cronk, 1980) (W.J. Hooker) EX EX Last seen 1958 (Duffey, 1964) De Vriese Rare NT Localized population but no immediate Kunth EN VU (D2) Highly localized small population; S. Watson Rare NT Localized but large population; no (Hook) O. EN EX Last seen 1889 (Cronk, 1980) L. Rare(B1 CR Swartz EN CR (B1) Highly localized small population; Brogn. (Poaceae) EN EX Last seen 1886 (Cronk, 1980) Species endemic to Ascension Island, with their original Red List status (Cronk, 1980), updated and criteria (s EX, Extinct in the wild; CR, Critically Endangered; EN, VU, Vulnerable; NT, Near Threatened Table 1 1 Species (Family) List status tion for the reassessment, estimated population size, number of localities in which found and these where severe thre and total number of associated native species exotic recorded from all localities where endemics were fo Euphorbia origanoides Sporobolus caespitosus (Euphorbiaceae)(Poaceae)Sporobolus durus Oldenlandia adscensionis Cronk (Rubiaceae) Asplenium ascensionis decline over last 50 years but probable decline over last 150 years Diels (Adiantaceae) Pteris adscensionis (Aspleniaceae)Anogramma ascensionis (Adiantaceae)Dryopteris ascensionis Kuntze (Dryopteridaceae) Xiphopteris ascensionensis immediate threats but may be prone to severe decline over last 150 years (Hieronymus) Cronk(Grammitidaceae)Marattia purpurascens (Marattiaceae) immediate threats but may be prone to long-term threats such as habitat threats;

Table 2 Severe threat species (see text for details) and whether they are present (P) at locations where the six extant endemic species (Table 1) occur.

A. M. E. P. S. X. Species ascensionis purpurascens origanoides adscensionis caespitosus ascensionensis

Alpinia zerumbet (Pers.) B.L.Burtt & R.M.Smith P Heliotropium sp. P Juniperus bermudiana L. P P Lantana camara L. P Leucaena leucocephala (Lam.) de Wit P Melinis minutiflora Beauv. P P Nicotiana glauca R.Grah. P P Opuntia sp. P P Paspalum conjugatum Berg. P P P Paspalum scrobiculatum L. P P P Prosopis juliflora (Sw.) DC P Psidium guajava L. P P P P P Sporobolus africanus (Poir.) Robyns & Tournay P P P

This plant inhabits near vertical cliff faces and popula- advocated by Cronk (2000) could be extended to all tions are therefore potentially susceptible to catastrophic endemic species. The newly created Governmental events. We have categorized E. origanoides as Critically Organiation, Ascension Island Conservation, has Endangered because of the presence of P. juliflora and a recently gained funding to develop Green Mountain as a highly localized distribution and fluctuating population National Park, a key component of which is the creation size, which increase susceptibility to catastrophic events. of a nursery, the propagation of endemic plants and trial Because of the low species diversity of Ascension, reintroductions. Cronk (1980) suggested that the endemic plant species are poor competitors. Although this assumption has not been explicitly tested, the ecological changes on Ascen- Acknowledgements sion Island in recent times support this contention We would like to thank Lucinda Kirk, Paul Robinson, (Ashmole & Ashmole, 2000) and both introduced plants Zoe Smolka, Lucy Webster, Sam Gardner, Richard and feral animals remain the most pervasive threats. White, Dale Hawkins, Mike Bell and Dave Boyle for There has been an increase from 25 native species to conducting fieldwork, and Colin Legg, Claire Pannell, 200–300 vascular plant species in c. 100 years (Ashmole Shahina A. Ghazanfar and an anonymous reviewer for & Ashmole, 2000) and this has had disastrous conse- comments on the manuscript. Funding for field work quences for the native plants (Duffey, 1964; Cronk, 1980, was provided by the Weir Fund for Field Studies, the 2000; Ashmole & Ashmole, 2000). Although mass Davis Fund, a BP Conservation Award, James Rennie introductions have ceased, plants are still colonizing the Bequest, and the USAF on Ascension funded work on island. Recent additions include P. juliflora, an extremely E. origanoides. successful colonizer of the lava plains, and a species of Heliotropium that has rapidly colonized many coastal areas since its introduction in the 1990s. The 1958 map References (Duffey, 1964) of the vegetation is now out of date for Ashmole, P. & Ashmole M. (2000) St Helena and Ascension many areas, particularly with regard to P. juliflora. With- Island: A Natural History. Anthony Nelson, Oswestry, UK. out control measures the likelihood of more problem Cronk, Q.C.B. (1980) Extinction and survival in the endemic species arriving remains high. Funding is being sought flora of Ascension Island. Biological Conservation, 17, 207–219. for a regional invasive species control programme that Cronk, Q.C.B. (2000) The Endemic Flora of St Helena. Anthony will build the infrastructure necessary to control the Nelson, Oswestry, UK. Cronk, Q.C.B. & Fuller, J.L. (2000) Plant Invaders: The Threat to imports of invasive plant species. Natural Ecosystems. 2nd Edition. Chapman and Hall, London, The future of the endemic vascular flora of Ascension UK. remains uncertain. Cronk (2000) stated that, unlike St Cunninghamme, J. (1699) A catalogue of shells etc., gathered at Helena, there is little that can be done for plant conserva- the island of Ascension by Mr. James Cunninghame. tion on Ascension as endemics have either successfully Philosophical Transactions of the Royal Society, 21, 295–300. Duffey, E. (1964) The terrestrial ecology of Ascension Island. adapted or gone extinct. However, the continuing and Journal of Applied Ecology, 1, 219–251. increasing presence of introduced species continue to Fowler, S.V. (1998) Report on the Invasion, Impact and Control of pose new threats. The cultivation of Pteris adscensionis ‘Mexican Thorn’, Prosopis juliflora, on Ascension Island.

Unpublished Report, International Institute of Biological Walter, K.S. & Gillett, H.J. (eds) (1998) 1997 IUCN Red List of Control, Ascot, UK. Threatened Plants. IUCN, Gland, Switzerland and Cambridge, Gray, A. (2003) Aspects of the Ecology of Euphorbia origanoides UK. L. Unpublished Report for USAF, Ascension Island. Weber, E. (2003) Invasive Plant Species of the World. CABI Gray, A. (2004) The parable of Green Mountain: massaging the Publishing, Wallingford, UK. message. Journal of Biogeography, 31, 1549–1550. Wilkinson, D.M. (2004) The parable of Green Mountain: Gray A., Gardner S., Kirk L., Robinson P., Smolka Z. & Webster Ascension Island, ecosystem construction and ecological L. (2000) The Status and Distribution of the Endemic Vascular fitting. Journal of Biogeography, 31, 1–4. Flora of Ascension Island. Unpublished Report, University of Edinburgh, UK. Hooker, J.D. (1867) On insular floras. Gardeners Chronicle Appendices 1–2 (British Association 1866). IPNI (2004) The International Plant Names Index. The appendices for this article are available online at Http://www.ipni.org [accessed 15 August 2005]. http://journals.cambridge.org IUCN (2001) 2001 Categories and Criteria (version 3.1). IUCN, Gland, Switzerland [http://www.redlist.org/info/ categories_criteria2001.html, accessed 6 May 2005]. Biographical sketches Nielson, D.L. & Sibbett, B.S. (1996) Geology of Ascension Island, South Atlantic Ocean. Geothermics, 45, 427–448. Packer, J.E. & Packer, L. (1998) Contributions Towards a Flora of Alan Gray is currently researching management of vegeta- Ascension Island. Ascension Heritage Society, Georgetown, tion and carbon fluxes in peatland ecosystems in the UK. He Ascension Island. has conducted research on the endemic vascular flora of Pearce, F. (2004) The accidental rainforest. New Scientist, 2465, Ascension Island since 1998. 44–45. Tara Pelembe carries out research on the conservation of Pickup, A.R. (1999) Ascension Island Management Plan. Ascension’s biodiversity, including seabirds, marine turtles Unpublished Report, Royal Society for the Protection of and vegetation. Birds, Sandy, UK. Royal Botanic Gardens Kew (1999) Survey of Economic Plants for Stedson Stroud’s research interests are in the practical con- Arid and Semi-Arid Lands (SEPASAL) Database. servation of the endemic flora of Ascension Island, including Http://www.rbgkew.org.uk/ceb/sepasal/internet/ techniques for the restoration of endemic plant populations. [accessed 6 May 2005].